1. Field of the Invention
The present invention relates to methods and apparatus for cooling panel structures, and more particularly to a method and apparatus for the cooling or aircraft structures located rearwardly of the engine bay doors in the flow of hot, corrosive jet engine exhaust gases.
2. Background of the Invention
Typically, exhaust impinged aircraft structures located rearwardly of engine bays in the flow of hot, corrosive jet engine exhaust are subjected to acoustic, pressure and thermal loads not ordinarily encountered in either conventional airframe designs or in engines.
The design of exhaust impinged structures, contrasted with hot sections of engines, is dependent on thermal and geometric constraints. For example, exhaust-impinged structures are typically exposed to temperatures in the range of from about 1000.degree. F. for a bomber to about 3000.degree. F. for a fighter. Moreover, engine sections can generally be designed to minimize the thermal stresses, while design of the hot exhaust-impinged structures have configurations which rely on other considerations, such as infrared signature (IR) or Radar Cross Section (RCS) constraints. And as a rule, exhaust-impinged structures are required to conform to the surrounding aircraft structure. Such constraints increase the thermal stresses that lead to failure. Non-uniform heating and relatively low thermal conductivity aggrevate the thermal stress distribution, and additional problems are encountered, including low-cycle fatigue and increases in the mean stresses and buckling.
Exhaust-impinged structures are used in a complex and demanding service environment. At the surface exposed to the engine exhaust stream, the environment may be oxygen-depleted with respect to air, but it is still an oxidizing environment. Fuel additives may increase the corrosivity of the exhaust stream for the particular propulsion system. The temperature of the aft deck substructure varies from the ambient air temperature before takeoff to the temperature of any cooling air used or to temperatures approaching those of an uncooled deck, and therefore significant thermal gradients occur within the structure.
Material characteristics, too, place substantial constraints on the aircraft engine and airframe system integration capability. For example, without cooling, the baseline material typically used, Ti-6A1-4V, has insufficient high temperature strength, creep resistance, fatigue resistance and resistance to cyclic oxidation. The fatigue resistance of a structure cycled to high temperatures, with uneven heating, is related to the coefficient of thermal expansion (CTE), and a high CTE translates into greater thermal strains and thus stresses for a given temperature change.
It has therefore become necessary to find appropriate mechanisms for controlling temperature changes in these materials at these surface areas, as well as for cooling such surface areas.
Various systems and devices have been used to assist in reducing impingement temperatures, including heat pipes and backside cooling using fluids such as water, engine fan air, or compressed air.
Heat pipes are unsatisfactory because the hot structure temperatures reduction are typically very small. Heat pipes employ cyclical evaporation and condensation of a working fluid in an elongated closed system. The working fluid is generally an organic heat transfer fluid or a metal. Incorporating heat pipes into an exhaust-impinged structure is not a viable option based on current technologies. The level of heat flux that the heat pipe would be required to handle would result in a large, heavy condensing system. Additionally, the available working fluids are toxic and corrosive, requiring expensive, heavy materials for containment, which increases the complexity of field repairs and maintainability.
Backside cooling involves the flow of a fluid beneath the surface of an exhaust-impinged structure to conduct heat away. The heat transfer capability is dependent upon the temperature and flow rate of the fluid, as well as the thermal conductivity of the wall material. For example, the flow of engine bleed air below the hot surface would not, by itself, be an efficient cooling medium. The cooling efficiency could be increased by using inlet air, which is cooler than bleed air, and by the incorporation of cooling fins on the backside to maximize heat transfer. However, the cooling fins also lead to an increased rate of heat transfer into the cooling air, and as the air is heated, additional air must be drawn in to port the hot air away from the surface as it approaches the temperature of the deck surface. Although these methods may minimize thermal gradients and achieve a more uniform thermal profile across the sufrace, the overall temperature reduction of the hot structure is minimal.
Another arrangement that has been tried involves the isolation of susbstructure from the exhaust-impinged hot deck by attachment of the substructure with fasteners that "float" as the deck expands when the temperature increases. These floating surfaces reduce thermal loads transmitted to the substructure and relieve some of the loads induced by thermal expansion of the impinged surface.
Still another arrangement involves the attachment of a truss structure 20 to a separate cooling panel 10 (see FIG. 1 of the drawings) which forms the outer surface of the aircraft. This arrangement includes a cooling panel 12 secured atop the truss structure 20 via connecting hardware 16 and fastening elements 17 in such a manner as to form first chambers 18 between the upper surface 23 of the truss structure 20 and the lower surface 13 of the cooling panel. Second chambers 25 in the truss structure 20 carry pressurized fluid from a source to the chambers 18 via openings 21 in the portion of the truss structure which overlies the chambers 25. The cooling panel 10 is formed with a first set of openings 22 formed at the lower surface 13, and a second set of openings 24 formed at the upper surface 14 of the panel. Each opening in the first set constitutes a nozzle and has an axis of symmetry arranged substantially normal to the lower surface of the panel 10. Each opening of the second set also constitutes a nozzle and has an axis of symmetry disposed at an angle to the cooling panel upper surface 14. The two sets of openings fluidly communicate with each other through the thickness of the cooling panel, as well as with the fluid environments adjacent the upper and lower surfaces of the panel 12.
Pressurized fluid flowing into the second chambers 25 is forced through openings 21 into the first chambers 18, then through the first set of openings 22 causing a cooling of the panel 12, and then through the second set of openings 24 to create a film of cooling fluid flowing across the panel top surface.
None of the known concepts are workable; either they are excessively heavy due to part of the structures being non-load bearing or they do not incorporate impingement cooling and therefore require excessive air flow.